1. Field of the Invention
The present invention relates to a tire pressure monitoring system and a pressure monitoring unit for monitoring an air pressure of a tire mounted on a wheel of, for example, a vehicle.
2. Description of Related Art
Installation of a tire pressure monitoring system (hereinafter, referred to as TPMS) which immediately notifies the driver in a running vehicle that a pressure of the tire is low, in a new vehicle is already a requirement in North America, and the TPMS is expected to be common in the other areas of the world. As a method for realizing the TPMS, there are two methods. One is a direct method (sensor method) which directly detects an air pressure using an air pressure sensor, and the other is an indirect method which indirectly detects the air pressure based on a difference of a rotation speed of a wheel. The TPMS in this specification is the direct method.
Generally, the TPMS consists of a pressure sensor unit mounted on each wheel, a monitoring unit mounted on a vehicle body and a display unit mounted on a part of an instrument panel. The pressure sensor unit mounted on the wheel includes at least an air pressure sensor and a wireless communication device, detects an air pressure in a tire and transmits data of the detected air pressure to the monitoring unit. The monitoring unit monitors data of the air pressure transmitted from each air pressure sensor unit, and if the air pressure is lowered less than a predetermined value, the monitoring unit transmits display data indicating the lowering of the tire pressure to the display unit.
Concerning this kind of TPMS, a tire pressure monitoring apparatus is disclosed in, for example, Japanese Patent No. 3815305, where when one running wheel of a vehicle is replaced with a spare wheel on which a spare tire is mounted, an identification number (hereinafter, referred to as ID number) of the wheel already being identified as a running wheel is compared with the ID number of the wheel included in a signal containing a rotation detection signal, and if both the ID numbers are not matched, the not-matched ID numbers are exchanged to each other, so that a switching of an identification of the ID number (hereinafter, referred to as ID number identification) of the running wheel can be automatically executed.
However, in the tire pressure monitoring apparatus disclosed in Japanese Patent No. 3815305, when a vehicle is running carrying a spare wheel on which a tire pressure sensor is mounted, a switch of a centrifugal force sensor which detects a rotation of a tire, of a pressure sensor unit fixed on the spare wheel may be turned on by vibrations due to, for example, a rough road, and thereby a signal including an ID number may be transmitted from the pressure sensor unit. Therefore, there is a possibility that the ID number of the running wheel is miss-identified.
In addition, in the tire pressure monitoring apparatus disclosed in Japanese Patent No. 3815305, one spare wheel is assumed as a non-mounted wheel which is different from a running wheel. Therefore, it is difficult to automatically switch the ID number identification of the running wheel in the case that four wheels, each mounting a normal tire, which are commonly used in the vehicle are all replaced with the wheels of a winter tire, such as a studless tire or a snow tire.
In addition, in the tire pressure monitoring apparatus disclosed in Japanese Patent No. 3815305, when the wheel is replaced, it is difficult to change a check threshold value for checking a lowering of the air pressure to be used for the monitoring unit, in response to a difference of a set pressure value in a tire of a wheel before and after the replacement of the wheel.
The present invention has been developed in consideration of the forgoing problems, and it is an object of the present invention to provide a tire pressure monitoring system which can automatically switch the ID number identification of the running wheel even in the case that all running wheels are replaced at the same time, while preventing the ID number (sensor ID) of the pressure sensor of the running wheel from miss-identifying, and in addition, can change the check threshold value for checking a lowering of the air pressure to be used for the monitoring unit in response to the set pressure value of a new running wheel after the replacement.
In addition, it is another object of the present invention to provide a pressure monitoring unit which prepares a plurality of mounting wheel sets on each of which the pressure sensor unit is mounted, which selects one set from the prepared plurality of the mounting wheel sets as a running wheel set, which selects later a set which is not selected and ready and waiting as a running wheel, which can replace a current running wheel set with another wheel set by the wheel set, and which can monitor the air pressure using the set pressure value of the wheel set to be mounted by the replacement of the wheel set.